Molybdenum-containing alloys and associated systems and methods

ABSTRACT

Molybdenum-containing alloys, and associated systems and methods, are generally described. In certain embodiments, secondary and/or tertiary elements can be included, along with molybdenum, to provide beneficial properties during the sintering of the molybdenum-containing alloy. The molybdenum-containing alloys are, according to certain embodiments, nanocrystalline. According to certain embodiments, the molybdenum-containing alloys have high relative densities. The molybdenum-containing alloys can be relatively stable, according to certain embodiments. Inventive methods for making molybdenum-containing alloys are also described herein.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/968,233, filed Jan. 31, 2020, and entitled “Molybdenum-Containing Alloys and Associated Systems and Methods,” which is incorporated herein by reference in its entirety for all purposes.

GOVERNMENT SPONSORSHIP

This invention was made with Government support under Grant No. 80NSSC19K1055 by NASA Marshall Space Flight Center. The Government has certain rights in the invention.

TECHNICAL FIELD

Molybdenum-containing alloys and associated systems and methods are generally described.

SUMMARY

Molybdenum-containing alloys, and associated systems and methods, are generally described. In certain embodiments, secondary (and, optionally, tertiary) elements can be included, along with molybdenum, to provide beneficial properties during the sintering of the molybdenum-containing alloy. The molybdenum-containing alloys are, according to certain embodiments, nanocrystalline. According to certain embodiments, the molybdenum-containing alloys have high relative densities. The molybdenum-containing alloys can be relatively stable, according to certain embodiments. Inventive methods for making molybdenum-containing alloys are also described herein. The subject matter of the present invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.

Certain aspects are related to a method of forming a metal alloy. In some embodiments, the method comprises sintering particles comprising molybdenum (Mo) and a second element to produce the metal alloy, wherein Mo is the most abundant element by atomic percentage in the metal alloy, and the metal alloy has a relative density of at least 80%.

In some embodiments, the method comprises sintering particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy.

Metal alloys are also disclosed herein. In some embodiments, the metal alloy comprises molybdenum (Mo) and a second element, wherein Mo is the most abundant element by atomic percentage in the metal alloy, and the metal alloy has a relative density of at least 80%.

In certain embodiments, the metal alloy comprises molybdenum (Mo) and chromium (Cr), wherein Mo is the most abundant element by atomic percentage in the metal alloy.

Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.

FIGS. 1A-1C are exemplary schematic diagrams showing a sintering process, according to certain embodiments.

FIGS. 2A-2B show SEM images of an exemplary molybdenum-chromium alloy, according to one set of embodiments.

FIGS. 3A-3B show SEM images of an exemplary molybdenum-chromium-tungsten alloy, according to one set of embodiments.

FIG. 4 is, according to some embodiments, a diagram depicting the process of producing and sintering an alloy that exhibits nanophase separation sintering.

FIG. 5 is, in accordance with certain embodiments, an SEM image of powder after it has been mechanically alloyed but before it has been sintered.

FIG. 6 is, in accordance with some embodiments, an SEM image of Mo15Cr that has been sintered up to 1450° C. This sample has >98% relative density. The darker phase present is the chromium rich phase that assisted in sintering.

FIG. 7 is, in accordance with certain embodiments, an SEM image of Mo15Cr sintered to 1200° C. and quenched before full density was reached. In this figure, it is easier to see where the chromium rich phase (the darker material) formed necks between the particles.

FIG. 8 is, in accordance with certain embodiments, an SEM image of Mo25W15Cr that has been sintered to 1450° C. and achieved greater than 98% relative density. Similar to the Mo15Cr sample, the darker phase represents the chromium phase that formed to accelerate sintering.

FIG. 9 shows, in accordance with some embodiments, densification curves for the Mo15Cr alloy compared to pure Mo sintered at a heating rate of 10° C. per min to 1450° C.

FIG. 10 shows, in accordance with some embodiments, a densification curve for the Mo25W15Cr alloy, which was heated at a rate of 10° C. per min to 1450° C.

DETAILED DESCRIPTION

This disclosure is generally directed to metal alloys comprising molybdenum and methods of making molybdenum-containing alloys. Certain embodiments are related to making molybdenum-containing alloys via sintering. In certain embodiments, secondary and/or tertiary elements can be included, along with molybdenum, to provide beneficial properties during the sintering of the molybdenum-containing alloy. In certain cases, the molybdenum-containing alloys described herein comprise additional elements in addition to molybdenum, such as chromium (Cr) and/or tungsten (W). Other elements may also be present. In accordance with certain embodiments, the molybdenum-containing alloy described herein can contain at least three elements (e.g., at least three metal elements). The presence of three elements together is not, however, strictly required, and in other embodiments, the molybdenum-containing alloy may include only two elements.

As noted above, the present disclosure includes inventive methods for making molybdenum-containing alloys. For example, certain embodiments are directed to sintering methods in which the sintering is achieved at relatively low temperatures and/or over a relatively short period of time. In some embodiments, the sintering is performed with little or no applied pressure during the sintering process. According to some embodiments, and as described in more detail below, the sintering can be performed such that undesired grain growth is limited or eliminated (e.g., via the selection of materials and/or sintering conditions). Certain embodiments are directed to the recognition that one can sinter molybdenum-containing materials over relatively short times, at relatively low temperatures, and/or with relatively low (or no) applied pressure while maintaining high temperature stability, high relative density, and/or, in some cases, nanocrystallinity.

Certain of the embodiments described herein can provide advantages relative to prior articles, systems, and methods. For example, according to certain (although not necessarily all) embodiments, the molybdenum-containing metal alloys can have high strength, high hardness, and/or high resistance to grain growth. According to some (although not necessarily all) embodiments, the methods for forming metal alloys described herein can make use of relatively small amounts of energy, for example, due to the relatively short sintering times, the relatively low sintering temperatures, and/or the relatively low applied pressures that are employed.

In some embodiments, a metal alloy is formed by sintering a plurality of particles. The particles can be, in some embodiments, in the form of a powder. The shape of the particles may be, for example, spherical, cubical, conical, cylindrical, needle-like, irregular, or any other suitable geometry. In some embodiments, at least some (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) of the particles are single crystals. In certain embodiments, at least some (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) of the particles are polycrystalline.

The particles from which the metal alloy is formed can have any of a variety of sizes. In some embodiments, at least 50% (or at least 75%, at least 90%, at least 95%, or at least 99%) of the total particle volume is made up of particles having maximum cross-sectional dimensions of less than 1 millimeter (or less than 500 microns, less than 100 microns, or less than 10 microns).

FIGS. 1A-1C are exemplary schematic diagrams showing a sintering process, according to certain embodiments. In FIG. 1A, a plurality of particles 100 are shown in the form of spheres (although, as mentioned elsewhere, other shapes could be used). As shown in FIG. 1B, particles 100 can be arranged such that they contact each other. As shown in FIG. 1C, as the particles are heated, they agglomerate to form a single solid material 110. During the sintering process, according to certain embodiments, interstices 105 between particles 100 (shown in FIG. 1B) can be greatly reduced or eliminated, such that a solid having a high relative density is formed (shown in FIG. 1C).

According to certain embodiments, the particles from which the alloy is formed comprise a relatively large amount of molybdenum (Mo). For example, in some embodiments, Mo is the most abundant element (e.g., the most abundant metal) by atomic percentage in the particles. (Atomic percentages are abbreviated herein as “at. %” or “at %”.) According to certain embodiments, Mo is present in the particles in an amount of at least 50 at %, at least 55 at %, at least 60 at %, at least 65 at %, at least 70 at %, at least 80 at %, at least 90 at %, or at least 95 at %. In some embodiments, Mo is present in the particles in an amount of up to 96 at %, up to 97 at %, up to 98 at %, up to 99 at %, up to 99.5 at %, or more. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least some of the particles comprise Mo and/or a second element (e.g., a second metal). The phrase “second element” is used herein to describe any element that is not Mo. The phrase “second metal” is used herein to describe any metal element that is not Mo. The term “element” is used herein to refer to an element as found on the Periodic Table. “Metal elements” are those found in Groups 1-12 of the Periodic Table except hydrogen (H); Al, Ga, In, Tl, and Nh in Group 13 of the Periodic Table; Sn, Pb, and Fl in Group 14 of the Periodic Table; Bi and Mc in Group 15 of the Periodic Table; Po and Lv in Group 16 of the Periodic Table; the lanthanides; and the actinides.

In some embodiments, one portion of the particles is made up of Mo while another portion of the particles is made up of the second element (e.g., a second metal, such as chromium). In certain embodiments, at least some of the particles comprise both Mo and the second element (e.g., a second metal, such as chromium).

According to certain embodiments, the second element is selected from the group consisting of chromium (Cr) and palladium (Pd). In some embodiments, both Cr and Pd are present (e.g., in cases in which the particles comprises at least three elements). In other embodiments, only one of Cr and Pd is present. In some embodiments, the second element is Cr.

According to certain embodiments, the second element and Mo exhibit a miscibility gap. Two elements are said to exhibit a “miscibility gap” when the phase diagram of those two elements includes a region in which the mixture of the two elements exists as two or more phases. In some embodiments in which the second element and Mo exhibit a miscibility gap, the second element and Mo can be present in the metal alloy among at least two phases.

In some embodiments, the second element has a melting point that is lower than the melting point of molybdenum (Mo). As would be understood by a person of ordinary skill in the art, the melting point of an element refers to the melting point of that element in its pure form. In the case of a metal, for example, the melting point of the metal refers to the melting point of that metal in its pure form.

In some embodiments, Mo is at least partially soluble in the second element. The second element (e.g., chromium, palladium) may be present in the particles from which the alloy is made in a variety of suitable percentages. According to certain embodiments, the second element is present in the particles in an amount of less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at %. In some embodiments, the second element is present in the metal alloy in an amount of at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 6 at %, at least 7 at %, at least 8 at %, at least 9 at %, at least 10 at %, or more. Combinations of these ranges are also possible. For example, in some embodiments, the second element is present in the metal alloy in an amount of from 0.5 at % to 40 at % of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount of from 1 at % to 40 at % of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount of from 8 at % to 32 at % of the metal alloy. Other values are also possible.

In some embodiments, the second element may be an activator element, relative to Mo. Activator elements are those elements that increase the rate of sintering of a material, relative to sintering rates that are observed in the absence of the activator element but under otherwise identical conditions. Activator elements are described in more detail below.

According to certain embodiments, the second element (e.g., for forming an alloy with Mo) can be selected based on one or more of the following conditions:

1. thermodynamic stabilization of the nanocrystalline grain size;

2. phase separation region, which is extended above the sintering temperature;

3. second (e.g., solute) element with lower melting temperature; and/or

4. solubility of the Mo in the precipitated second phase.

According to some embodiments, the second element (e.g., Cr) forms precipitates within the Mo parent phase. For example, in some embodiments, the metal alloy comprises a structure consisting of Mo-rich grains and Cr-rich precipitates. In some embodiments, the precipitates of the second element (e.g., Cr) can populate the grain boundaries between Mo grains. In some embodiments, the second metal is chromium. The inventors have recognized and appreciated within the context of the present disclosure that the addition of a second metal, such as chromium, may provide certain benefits when alloyed with molybdenum. Without wishing to be bound my theory, is it believed that chromium may segregate into a secondary phase upon heating and also possesses a lower surface energy than molybdenum and may preferentially segregates to the surface of an alloy to form bridges or “necks” between grains of molybdenum. That is to say, chromium may form connecting linkages at the grain boundaries of molybdenum particles within the alloy. In addition, chromium may also allow rapid diffusion of molybdenum through the chromium necks to promote rapid densification. For example, in FIGS. 2A-2B, SEM images of an exemplary Mo15Cr are shown that capture different stages of neck formation. In FIG. 2A, the Mo15Cr alloy has been heated to 850° C. and quenched and shows the early stages of neck formation. In FIG. 2B, the Mo15Cr sample has been heated to 1200° C. and quenched and shows intermediate neck growth and densification. Likewise, FIGS. 3A-3B show SEM images of an exemplary Mo25W15Cr alloy. In FIG. 3A, the Mo25W15Cr has been heated to 900° C. and quenched and shows early stages of neck formation. In FIG. 3B, the Mo25W15Cr alloy has been heated to 1200° C. and quenched and shows intermediate neck growth and densification.

In some embodiments, the particles from which the metal alloy is formed contains only Mo and the second element (i.e., Mo and the second element without additional metals or other elements). In other embodiments, the particles comprise Mo, the second element, and a third element. For example, in some embodiments, the particles comprise a third element (in addition to Mo and the second element). The third element can be, in some embodiments, a metal element. The phrase “third element” is used herein to describe an element that is not Mo and that is not the second element. That is to say, the third element, when present, is different from Mo and the second element. In some embodiments, the metal alloy comprises a third metal, in which case the alloy comprises Mo, a second metal, and a third metal.

In certain embodiments, the particles from which the alloy is formed (e.g., containing molybdenum, a second metal, and an optional third or additional metal) may contain a relatively large amount of metallic material. In some embodiments, at least 10 at %, at least 20 at %, at least 40 at %, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the particle material is made up of metal atoms in their metallic form (i.e., in an oxidation state of zero). In some embodiments, at least 10 at %, at least 20 at %, at least 40 at %, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the molybdenum atoms within the particles are in their metallic form. In certain embodiments, at least 10 at %, at least 20 at %, at least 40 at %, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the second element (e.g., second metal) atoms within the particles are in their metallic form. In some embodiments, at least 10 at %, at least 20 at %, at least 40 at %, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the third element (e.g., third metal) atoms within the particles are in their metallic form. In some embodiments, the molybdenum atoms can form metallic bonds with other neighboring atoms, such as other molybdenum atoms and/or atoms of the second element (e.g., second metal) and/or the third element (e.g., third metal).

According to certain embodiments, the third element is selected from the group consisting of tungsten (W) and tantalum (Ta). In some embodiments, the third element is W.

According to some embodiments, the third element, when present, and the second element exhibit a miscibility gap. In some embodiments in which the third element and the second element exhibit a miscibility gap, the third element and the second element can be present in the metal alloy among at least two phases.

In some embodiments, the third element (e.g., W, Ta) may increase the melting temperature of certain of the Mo-based alloys described herein. For example, tungsten has a high melting temperature and forms a solid solution with molybdenum. It is believed that, because of this, the melting temperature of the alloy may be selectively tuned by increasing the quantity of tungsten in the Mo-based alloy. As a non-limiting example, an alloy that comprises 60 at % molybdenum, 25 at % tungsten, and 15 at % chromium (Mo25W15Cr) may exhibit a melting temperature 100 degrees (° C.) higher than pure molybdenum.

In some embodiments, the third element has a melting point that is lower than the melting point of molybdenum (Mo).

The third element (e.g. tungsten) may be present in the particles in a variety of suitable percentages. According to certain embodiments, the third element is present in the particles in an amount of less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 30 at %, less than or equal to 28 at %, less than or equal to 26 at %, or less. In some embodiments, the third element is present in the metal alloy in an amount of at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 10 at %, at least 15 at %, at least 20 at %, at least 22 at %, at least 24 at %, or more. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, the total amount of all metal elements in the particles that are not Mo (e.g., the second element, the third element, and any additional optional elements) makes up less than 50 at %, less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at % of the particles. In some embodiments, the total amount of all elements in the particles that are not Mo (e.g., the second element, the optional third element, and any additional optional elements) makes up at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 8 at %, at least 10 at %, at least 12 at %, at least 14 at %, or more of the particles. Combinations of these ranges are also possible. Other values are also possible.

In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particles is less than 50 at %, less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at % of the particles. In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particles is at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 8 at %, at least 10 at %, at least 12 at %, at least 14 at %, or more. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the particles is from 0.5 at % to 50 at % of the particles. In some of these embodiments, at least 90 at % (or at least 95 at %, at least 98 at %, at least 99 at %, or at least 99.9 at %) of the balance of the particles is made of molybdenum.

Those of ordinary skill in the art would understand that, to determine the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in a given set of particles, one would sum the atomic percentages of each of these elements. For example, if the particles contain 60 at % Mo, 15 at % Cr, and 25 at % W, then the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present would be 40 at % (i.e., 15 at % from Cr, 25 at % from W, and 0 at % for all other elements in the list). Those of ordinary skill in the art would also understand that, in making this calculation, not all of the elements in the list above would necessarily be present in the particles. In the exemplary calculation described above, for example, palladium and tantalum are not present in the particles.

In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the particles is less than 50 at %, less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at % of the particles. In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the particles is at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 8 at %, at least 10 at %, at least 12 at %, at least 14 at %, or more. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the particles is from 0.5 at % to 50 at % of the particles. In some of these embodiments, at least 90 at % (or at least 95 at %, at least 98 at %, at least 99 at %, or at least 99.9 at %) of the balance of the particles is made of molybdenum.

In some embodiments, the particles comprise Mo, Cr, and W. In some embodiments, the Mo is present in the particles in an amount of at least 50 at % (e.g., from 50 at % to 99 at %), the Cr is present in the particles in an amount of from 0.5 at % to 30 at %; and the W is present in the particles in an amount of from 0.5 at % to 30 at %. In some embodiments, the W is present in the particles in an amount of from 20 at % to 30 at %; the Cr is present in the particles in an amount of from 10 at % to 20 at %; and at least 90 at % (or at least 95 at %, at least 98 at %, at least 99 at %, or at least 99.9 at %) of the balance of the particles is Mo. In some embodiments, the Mo is present in the particles in an amount of from 50 at % to 70 at %, the W is present in the particles in an amount of from 20 at % to 30 at %, and the Cr is present in the particles in an amount of from 10 at % to 20 at %.

The particles that are sintered can be, according to certain embodiments, nanocrystalline particles. The nanocrystalline particles can comprise, according to certain embodiments, grains with a grain size smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 40 nm, smaller than or equal to 30 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm. According to certain embodiments, at least some of the nanocrystalline particles have a grain size of smaller than or equal to 10 nm. In some embodiments, at least some of the nanocrystalline particles have a grain size of greater than or equal to 5 nm and smaller than or equal to 25 nm. In some embodiments, at least some of the nanocrystalline particles have a grain size of greater than or equal to 10 nm and smaller than or equal to 20 nm.

According to certain embodiments, at least some of the nanocrystalline particles comprise Mo, a second element (e.g., a second metal, such as chromium), and/or a third element (e.g., a third metal, such as tungsten). In some embodiments, one portion of the nanocrystalline particles is made up of Mo, while another portion of the nanocrystalline particles are made up of the second element, while yet another portion of the nanocrystalline particles are made up of the third element. In certain embodiments, at least some of the nanocrystalline particles comprise both Mo and the second element. In certain embodiments, at least some of the nanocrystalline particles comprise both Mo and the third element. In certain embodiments, at least some of the nanocrystalline particles comprise Mo, the second element, and the third element.

In some embodiments, Mo is the most abundant element by atomic percentage in at least some of the nanocrystalline particles. In some embodiments, Mo is the most abundant metal by atomic percentage in at least some of the nanocrystalline particles. In some embodiments, Mo is the most abundant metal element by atomic percentage in at least some of the nanocrystalline particles. In some embodiments, at least some of the particles contain Mo in an amount of at least 50 at %, at least 55 at %, at least 60 at %, at least 70 at %, at least 80 at %, at least 90 at %, or at least 95 at %. In some embodiments, at least some of the particles contain Mo in an amount of up to 96 at %, up to 97 at %, up to 98 at %, or more. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least some of the particles are formed by mechanically working a powder comprising the Mo and the second element. For example, certain embodiments comprise making particles, at least in part, by mechanically working a powder including a plurality of Mo particles and a plurality of second element particles (e.g., particles comprising Cr). Certain embodiments comprise making particles, at least in part, by mechanically working particles that include both Mo and the second element.

According to certain embodiments, at least some of the particles are formed by mechanically working a powder comprising the Mo, the second element (e.g., chromium), and the third element (e.g., tungsten). For example, certain embodiments comprise making particles (e.g., nanocrystalline particles), at least in part, by mechanically working a powder including a plurality of Mo particles, a plurality of second element particles (e.g., particles comprising Cr), and a plurality of third element particles (e.g., particles comprising W). Certain embodiments comprise making particles (e.g., nanocrystalline particles), at least in part, by mechanically working particles that include both Mo and the second element; both Mo and the third element; both the second element and the third element; and/or all of Mo, the second element, and the third element.

In embodiments that make use of mechanical working, any appropriate method of mechanical working may be employed to mechanically work a powder and form particles. According to certain embodiments, at least some of the particles are formed by ball milling a powder comprising the Mo and the second element (and/or, when present, the third element). The ball milling process may be, for example, a high energy ball milling process. In a non-limiting exemplary ball milling process, a tungsten carbide or steel milling vial may be employed, with a ball-to-powder ratio of 2:1 to 20:1 (e.g., from 5:1 to 12:1, such as 10:1), and an ethanol process control agent content of 0.01 to 3 mg/g of powder. According to certain other embodiments, the mechanical working is carried out in the absence of a process control agent. Other types of mechanical working may also be employed, including but not limited to, shaker milling and planetary milling. In some embodiments, the mechanical working (e.g., via ball milling or another process) may be performed under conditions sufficient to produce particles (e.g., nanocrystalline particles) comprising a supersaturated phase. Supersaturated phases are described in more detail below.

According to certain embodiments, the mechanical working (e.g., ball milling) is performed at a relatively low temperature. For example, in some embodiments, the mechanical working (e.g., ball milling) is performed while the particles are at a temperature of less than or equal to 150° C., less than or equal to 100° C., less than or equal to 75° C., less than or equal to 50° C., less than or equal to 40° C., less than or equal to 35° C., less than or equal to 30° C., less than or equal to 25° C., or less than or equal to 20° C. In some embodiments, the mechanical working (e.g., ball milling) is performed while the particles are at a temperature of at least 0° C. In some embodiments, the mechanical working (e.g., ball milling) is performed at a temperature of the surrounding, ambient environment.

In certain embodiments, the mechanical working (e.g., ball milling) may be conducted for a time of greater than or equal to 6 hours (e.g., greater than or equal to 8 hours, greater than or equal to 10 hours, greater than or equal to 12 hours, or greater than or equal to 15 hours). In certain embodiments, the mechanical working (e.g., ball milling) may be conducted for a time of less than or equal to 18 hours. In some embodiments, the mechanical working (e.g., ball milling) may be conducted for a time of 6 hour to 18 hours. In some cases, if the mechanical working time is too long, the Mo and/or the second element (and/or the third element, if present) may be contaminated by the material used to perform the mechanical working (e.g., milling vial material). The amount of the second element (and/or the third element, if present) that is dissolved in the Mo may, in some cases, increase with increasing mechanical working (e.g., milling) time. In some embodiments, after the mechanical working step (e.g., ball milling step), a phase rich in the second element material may be present.

According to certain embodiments, the Mo and the second element (and/or the third element, if present) are present in the particles in a non-equilibrium phase. The particles may, according to certain embodiments, include a non-equilibrium phase in which the second element (and/or the third element, if present) is dissolved in the Mo. In some embodiments, the non-equilibrium phase comprises a solid solution. According to some embodiments, the non-equilibrium phase may be a supersaturated phase comprising the second element (and/or the third element, if present) dissolved in the Mo. A “supersaturated phase,” as used herein, refers to a phase in which a material is dissolved in another material in an amount that exceeds the solubility limit. The supersaturated phase can include, in some embodiments, an activator element and/or a stabilizer element forcibly dissolved in the Mo in an amount that exceeds the amount of the activator element and/or the stabilizer element that could be otherwise dissolved in an equilibrium phase of the Mo. For example, in one set of embodiments, the supersaturated phase is a phase that includes an activator element forcibly dissolved in Mo in an amount that exceeds the amount of activator element that could be otherwise dissolved in an equilibrium Mo phase.

In some embodiments, the supersaturated phase may be the only phase present after the mechanical working (e.g., ball milling) process.

According to certain embodiments, the non-equilibrium phase may undergo decomposition during the sintering of the particles (which sintering is described in more detail below). The sintering of the particles may cause the formation of a phase rich in the third element at at least one of the surfaces and/or grain boundaries of the particles. In some such embodiments, the Mo is soluble in the phase rich in the second and/or the third element. The formation of the phase rich in the second and/or third element may be the result of the decomposition of the non-equilibrium phase during the sintering. The phase rich in the second and/or third element may, according to certain embodiments, act as a fast diffusion path for the Mo, enhancing the sintering kinetics and accelerating the rate of sintering of the particles. According to some embodiments, the decomposition of the non-equilibrium phase during the sintering of the particles accelerates the rate of sintering of the particles.

Certain, although not necessarily all, embodiments comprise cold pressing the plurality of particles during at least one portion of time prior to the sintering. It has been found that, according to certain embodiments, metal alloys comprising Mo and a second element (e.g., Mo and Cr), and/or metal alloys comprising Mo, a second element, and a third element (e.g., Mo, Cr, and W) can be compressed such that high relative densities are achieved without the need for simultaneous heating. In some embodiments, the cold pressing comprises compressing of the plurality of particles at a force greater than or equal to 300 MPa, greater than or equal to 400 MPa, greater than or equal to 500 MPa, greater than or equal to 750 MPa, greater than or equal to 1000 MPa, or higher. In some embodiments, the cold compression comprises compressing the plurality of particles at a force of up to 1400 MPa, or greater. Combinations of these ranges are also possible (e.g., greater than or equal to 300 MPa and less than or equal to 1400 MPa). Other ranges are also possible.

According to certain embodiments, the cold compression is performed at a relatively low temperature. For example, in some embodiments, the cold compression is performed while the particles are at a temperature of less than or equal to 150° C., less than or equal to 100° C., less than or equal to 75° C., less than or equal to 50° C., less than or equal to 40° C., less than or equal to 35° C., less than or equal to 30° C., less than or equal to 25° C., or less than or equal to 20° C. In some embodiments, the cold compression is performed at a temperature of the surrounding, ambient environment.

As noted above, certain embodiments comprise sintering a plurality of particles to form the metal alloy. Those of ordinary skill in the art are familiar with the process of sintering, which involves applying heat to the material (e.g., particles) that is to be sintered such that the material becomes a single solid mass.

According to certain embodiments, the sintering can be performed when the metal particles are at a relatively low temperature and/or for a relatively short period of time, while maintaining the ability to form metal alloys having high relative densities, small grain sizes, and/or equiaxed grains.

According to certain embodiments, sintering the plurality of particles involves heating the particles to a sintering temperature of less than or equal to 2200° C., less than or equal to 2000° C., less than or equal to 1900° C., less than or equal to 1800° C., less than or equal to 1700° C., less than or equal to 1600° C., less than or equal to 1500° C., less than or equal to 1400° C., less than or equal to 1300° C., less than or equal to 1200° C., less than or equal to 1100° C., less than or equal to 1000° C., less than or equal to 900° C., less than or equal to 850° C., less than or equal to 800° C., or less than or equal to 750° C. According to certain embodiments, sintering the plurality of particles involves heating the particles to a sintering temperature of greater than or equal to 750° C., greater than or equal to 850° C., greater than or equal to 1000° C., greater than or equal to 1200° C., greater than or equal to 1450° C., or greater than or equal to 1600° C. Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of particles involves heating the particles to a sintering temperature that is greater than or equal to 750° C. and less than or equal to 2200° C. In some embodiments, the temperature of the sintered material is within these ranges for at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or at least 99% of the sintering time.

According to certain embodiments, sintering the plurality of particles involves maintaining the particles within the range of sintering temperatures for less than 72 hours, less than 48 hours, less than or equal to 24 hours, less than or equal to 12 hours, less than or equal to 6 hours, less than or equal to 4 hours, less than or equal to 3 hours, less than or equal to 2 hours, or less than or equal to 1 hour (and/or, in some embodiments, for at least 10 minutes, at least 20 minutes, at least 30 minutes, at least 50 minutes, at least 3 hours, or at least 6 hours). Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of particles involves heating the particles to a first sintering temperature that is greater than or equal to 600° C. and less than or equal to 1100° C. for a sintering duration greater than or equal to 6 hours and less than or equal to 24 hours.

According to certain embodiments, sintering comprises heating the particles to a first sintering temperature that is lower than a second sintering temperature needed for sintering Mo in the absence of the second element. To determine whether such conditions were met, one of ordinary skill in the art would compare the temperature necessary to achieve sintering in the sample containing the Mo and the second element to the temperature necessary to achieve sintering in a sample containing the Mo without the second element, but otherwise identical to the sample containing the Mo and the second element. In some embodiments, the first sintering temperature can be at least 25° C., at least 50° C., at least 100° C., or at least 200° C. lower than the second sintering temperature.

According to certain embodiments, a non-equilibrium phase present in the particles (e.g., any of the non-equilibrium phases described above or elsewhere herein) undergoes decomposition during the sintering. In some such embodiments, the decomposition of the non-equilibrium phase accelerates a rate of sintering of the particles.

In some embodiments, the sintering further comprises forming a second phase at at least one of a surface and a grain boundary of the particles during the sintering. In some such embodiments, the second phase is rich in the second element. The term “rich” with respect to the content of an element in a phase refers to a content of the element in the phase of at least 50 at % (e.g., at least 60 at %, at least 70 at %, at least 80 at %, at least 90 at %, at least 99 at %, or higher). The term “phase” is generally used herein to refer to a state of matter. For example, the phase can refer to a phase shown on a phase diagram. Generally, when multiple phases are present, they are distinguishable from each other, even if both are solid phases.

The sintering may be conducted in a variety of suitable environments. In certain embodiments, the particles are in an inert atmosphere during the sintering process. The use of an inert atmosphere can be useful, for example, when reactive metals are employed in the particles. For example, Mo and Cr are reactive (separately and/or together) with oxygen.

In some embodiments, the sintering is performed in an atmosphere in which at least 90 vol. %, at least 95 vol. %, at least 99 vol. %, or substantially all of the atmosphere is made up of an inert gas. The inert gas can be or comprise, for example, helium, argon, xenon, neon, krypton, combinations of two or more of these, or other inert gas(es).

In certain embodiments, oxygen scavengers (e.g., getters) may be included in the sintering environment. The use of oxygen scavengers can reduce the degree to which the metals are oxidized during the sintering process, which may be advantageous according to certain embodiments. In some embodiments, the sintering environment can be controlled such that oxygen is present in an amount of less than 1 vol. %, less than 0.1 vol. %, less than 100 parts per million (ppm), less than 10 ppm, or less than 1 ppm.

In certain embodiments, the sintering is performed in an atmosphere containing a gas that, when exposed to oxygen gas (i.e., O₂) under the sintering conditions, will react with the oxygen gas. In some embodiments, the sintering is performed in an atmosphere comprising hydrogen gas (H₂). In some embodiments, the combination of hydrogen gas and inert gas makes up at least 90 vol. %, at least 95 vol. %, at least 99 vol. %, or substantially all of the atmosphere in which the sintering is performed. In some embodiments, the combination of hydrogen gas and argon gas makes up at least 90 vol. %, at least 95 vol. %, at least 99 vol. %, or substantially all of the atmosphere in which the sintering is performed.

According to certain embodiments, the sintering is conducted essentially free of external applied stress. For example, in some embodiments, for at least 20%, at least 50%, at least 75%, at least 90%, or at least 98% of the time during which sintering is performed, the maximum external pressure applied to the nanocrystalline particles is less than or equal to 2 MPa, less than or equal to 1 MPa, less than or equal to 0.5 MPa, or less than or equal to 0.1 MPa. The maximum external pressure applied to the nanocrystalline particles refers to the maximum pressure applied as a result of the application of a force external to the nanocrystalline particles, and excludes the pressure caused by gravity and arising between the nanocrystalline particles and the surface on which the nanocrystalline particles are positioned during the sintering process. Certain of the sintering processes described herein can allow for the production of relatively highly dense sintered ultra-fine and nanocrystalline materials even in the absence or substantial absence of external pressure applied during the sintering process. According to certain embodiments, the sintering may be a pressureless sintering process.

According to certain embodiments, at least one activator element may be present during the sintering process. The activator element may enhance the sintering kinetics of Mo. According to certain embodiments, the activator element may provide a high diffusion path for the Mo atoms. For example, in some embodiments, the activator element atoms may surround the Mo atoms and provide a relatively high transport diffusion path for the Mo atoms, thereby reducing the activation energy of diffusion of the Mo. In some embodiments, this technique is referred to as activated sintering. The activator element may, in some embodiments, lower the temperature required to sinter the nanocrystalline particles, relative to the temperature that would be required to sinter the nanocrystalline particles in the absence of the activator element but under otherwise identical conditions. Thus, the sintering may involve, according to certain embodiments, a first sintering temperature, and the first sintering temperature may be lower than a second sintering temperature needed for sintering the Mo in the absence of the third element. To determine the sintering temperature needed for sintering the Mo in the absence of the third element, one would prepare a sample of the Mo material that does not contain the third element but is otherwise identical to the nanocrystalline particle material. One would then determine the minimum temperature needed to sinter the sample that does not include the third element. In some embodiments, the presence of the second element lowers the sintering temperature by at least 25° C., at least 50° C., at least 100° C., at least 200° C., or more.

According to certain embodiments, at least one stabilizer element may be present during the sintering process. The stabilizer element may be any element capable of reducing the amount of grain growth that occurs, relative to the amount that would occur in the absence of the stabilizer element but under otherwise identical conditions. In some embodiments, the stabilizer element reduces grain growth by reducing the grain boundary energy of the sintered material, and/or by reducing the driving force for grain growth. The stabilizer element may, according to certain embodiments, exhibit a positive heat of mixing with the sintered material. The stabilizer element may stabilize nanocrystalline Mo by segregation in the grain boundaries. This segregation may reduce the grain boundary energy, and/or may reduce the driving force against grain growth in the alloy.

In some embodiments, the stabilizer element may also be the activator element. The use of a single element both as the stabilizer and activator elements has the added benefit, according to certain embodiments, of removing the need to consider the interaction between the activator and the stabilizer. In some embodiments, the element that may be utilized as both the activator and stabilizer element may be a metal element, which may be any of the aforedescribed metal elements.

According to certain embodiments, when one element cannot act as both the stabilizer and the activator, two elements may be employed. The interaction between the two elements may be accounted for, according to some embodiments, to ensure that the activator and stabilizer roles are properly fulfilled. For example, when the activator and the stabilizer form an intermetallic compound each of the elements may be prevented from fulfilling their designated role, in some cases. As a result, activator and stabilizer combinations with the ability to form intermetallic compounds at the expected sintering temperatures should be avoided, at least in some instances. The potential for the formation of intermetallic compounds between two elements may be analyzed with phase diagrams.

According to one set of embodiments, molybdenum particles and chromium particles (e.g., 10, 20, or 30 at % Cr with the balance being molybdenum) can be mechanically alloyed via ball milling, cold compressed, and subsequently annealed (e.g., in a thermomechanical analyzer for several hours). In some embodiments, the Mo—Cr alloy system exhibits nanocrystalline grain size stabilization by Cr segregation to Mo grain boundaries, and by formation of Cr-rich precipitates which pin grain boundaries and further prevent grain growth.

According to certain embodiments, powders of elemental Mo, Cr, and W are mixed and milled to achieve supersaturation and a decrease of the grain size to the nanometer scale. In some embodiments, annealing of compressed powders leads to the development of a nano-duplex structure consisting of Mo-rich grains and Cr-rich precipitates.

As noted above, certain embodiments are related to inventive metal alloys. The metal alloys comprise, according to certain embodiments, molybdenum and at least one other metal.

According to certain embodiments, the metal alloy comprises a relatively large amount of molybdenum (Mo). For example, in some embodiments, Mo is the most abundant element (e.g., the most abundant metal) by atomic percentage in the metal alloy. According to certain embodiments, Mo is present in the metal alloy in an amount of at least 50 at %, at least 55 at %, at least 60 at %, at least 65 at %, at least 70 at %, at least 80 at %, at least 90 at %, or at least 95 at %. In some embodiments, Mo is present in the metal alloy in an amount of up to 96 at %, up to 97 at %, up to 98 at %, up to 99 at %, up to 99.5 at %, or more. Combinations of these ranges are also possible. Other values are also possible.

The metal alloys described herein can comprise a second element. For example, the metal alloys described herein can comprise a second metal.

According to certain embodiments, the second element is selected from the group consisting of chromium (Cr) and palladium (Pd). In some embodiments, both Cr and Pd are present (e.g., in cases in which the alloy comprises at least three elements). In other embodiments, only one of Cr and Pd is present. In some embodiments, the second element is Cr.

In some embodiments, Mo is at least partially soluble in the second element. For example, in some embodiments, Mo and the second element are in a solid solution.

The second element (e.g., chromium, palladium) may be present in the metal alloy in a variety of suitable percentages. According to certain embodiments, the second element is present in the metal alloy in an amount of less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at %. In some embodiments, the second element is present in the metal alloy in an amount of at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 6 at %, at least 7 at %, at least 8 at %, at least 9 at %, at least 10 at %, or more. Combinations of these ranges are also possible. For example, in some embodiments, the second element is present in the metal alloy in an amount of from 0.5 at % to 40 at % of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount of from 1 at % to 40 at % of the metal alloy. In some embodiments, the second element is present in the metal alloy in an amount of from 8 at % to 32 at % of the metal alloy. Other values are also possible.

In certain embodiments, the metal alloy (e.g., containing molybdenum, a second metal, and an optional third or additional metal) may contain a relatively large amount of metallic material. In some embodiments, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the metal alloy is made up of metal atoms in their metallic form (i.e., in an oxidation state of zero). In some embodiments, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the molybdenum atoms within the metal alloy are in their metallic form. In certain embodiments, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the second element (e.g., second metal) atoms within the metal alloy are in their metallic form. In some embodiments, at least 50 at %, at least 70 at %, at least 90 at %, at least 95 at %, at least 99 at %, at least 99.9 at %, or more of the third element (e.g., third metal) atoms within the metal alloy are in their metallic form. In some embodiments, the molybdenum atoms can form metallic bonds with other neighboring atoms, such as other molybdenum atoms and/or atoms of the second element (e.g., second metal) and/or the third element (e.g., third metal).

In some embodiments, the metal alloy comprises only Mo and the second element (i.e., Mo and the second element without additional metals or other elements). In other embodiments, the metal alloy comprises Mo, the second element, and a third element. For example, in some embodiments, the metal alloy comprises a third element (in addition to Mo and the second element). The third element can be, in some embodiments, a metal element. In some embodiments, the metal alloy comprises a third metal, in which case the alloy comprises Mo, a second metal, and a third metal.

According to certain embodiments, the third element is selected from the group consisting of tungsten (W) and tantalum (Ta). In some embodiments, the third element is W.

The third element (e.g. tungsten) may be present in the metal alloy in a variety of suitable percentages. According to certain embodiments, the third element is present in the metal alloy in an amount of less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 30 at %, less than or equal to 28 at %, or less than or equal to 26 at %. In some embodiments, the third element is present in the metal alloy in an amount of at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 6 at %, at least 7 at %, at least 8 at %, at least 9 at %, at least 10 at %, or more. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, the total amount of all metal elements in the metal alloy that are not Mo (e.g., the second element, the third element, and any additional optional elements) makes up less than 50 at %, less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at % of the metal alloy. In some embodiments, the total amount of all elements in the metal alloy that are not Mo (e.g., the second element, the optional third element, and any additional optional elements) makes up at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 8 at %, at least 10 at %, at least 12 at %, at least 14 at %, or more. Combinations of these ranges are also possible. Other values are also possible.

In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is less than 50 at %, less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at % of the metal alloy. In some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 8 at %, at least 10 at %, at least 12 at %, at least 14 at %, or more. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in the metal alloy is from 0.5 at % to 50 at % of the metal alloy. In some of these embodiments, at least 90 at % (or at least 95 at %, at least 98 at %, at least 99 at %, or at least 99.9 at %) of the balance of the metal alloy is molybdenum.

Those of ordinary skill in the art would understand that, to determine the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present in a given metal alloy, one would sum the atomic percentages of each of these elements. For example, if the metal alloy contains 60 at % Mo, 15 at % Cr, and 25 at % W, then the total amount of chromium (Cr), palladium (Pd), tungsten (W), and tantalum (Ta) present would be 40 at % (i.e., 15 at % from Cr, 25 at % from W, and 0 at % for all other elements in the list). Those of ordinary skill in the art would also understand that, in making this calculation, not all of the elements in the list above would necessarily be present in the metal alloy. In the exemplary calculation described above, for example, palladium and tantalum are not present in the Mo—W—Cr alloy.

In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the metal alloy is less than 50 at %, less than or equal to 40 at %, less than or equal to 35 at %, less than or equal to 32 at %, less than or equal to 30 at %, less than or equal to 25 at %, less than or equal to 22 at %, less than or equal to 20 at %, less than or equal to 18 at %, or less than or equal to 16 at % of the metal alloy. In some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the metal alloy is at least 0.5 at %, at least 1 at %, at least 2 at %, at least 3 at %, at least 4 at %, at least 5 at %, at least 8 at %, at least 10 at %, at least 12 at %, at least 14 at %, or more. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of chromium (Cr) and tungsten (W) present in the metal alloy is from 0.5 at % to 50 at % of the metal alloy. In some of these embodiments, at least 90 at % (or at least 95 at %, at least 98 at %, at least 99 at %, or at least 99.9 at %) of the balance of the metal alloy is molybdenum.

In some embodiments, the metal alloy comprises Mo, Cr, and W. In some embodiments, the Mo is present in the metal alloy in an amount of at least 50 at % (e.g., from 50 at % to 99 at %), the Cr is present in the metal alloy in an amount of from 0.5 at % to 30 at %; and the W is present in the metal alloy in an amount of from 0.5 at % to 30 at %. In some embodiments, the W is present in the metal alloy in an amount of from 20 at % to 30 at %; the Cr is present in the metal alloy in an amount of from 10 at % to 20 at %; and at least 90 at % (or at least 95 at %, at least 98 at %, at least 99 at %, or at least 99.9 at %) of the balance of the metal alloy is Mo. In some embodiments, the Mo is present in the metal alloy in an amount of from 50 at % to 70 at %, the W is present in the metal alloy in an amount of from 20 at % to 30 at %, and the Cr is present in the metal alloy in an amount of from 10 at % to 20 at %.

The metal alloys comprising the molybdenum are, according to certain embodiments, nanocrystalline metal alloys. Nanocrystalline metals have certain advantages over their microcrystalline counterparts due to the large volume fraction of grain boundaries. As one example, nanocrystalline alloys generally have remarkably higher tensile strength.

Nanocrystalline materials generally refer to materials that comprise at least some grains with a grain size smaller than or equal to 1000 nm. In some embodiments, the nanocrystalline material comprises grains with a grain size smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, smaller than or equal to 10 nm, or smaller than or equal to 5 nm. In some embodiments, the nanocrystalline materials comprise grains with a grain size of at least 1 nm or at least 5 nm. Accordingly, in the case of metal alloys, nanocrystalline metal alloys are metal alloys that comprise grains with a grain size smaller than or equal to 1000 nm. In some embodiments, the nanocrystalline metal alloy comprises grains with a grain size smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm. In some embodiments, the nanocrystalline metal alloy comprises grains with a grain size of at least 1 nm, at least 2 nm, or at least 5 nm. Other values are also possible.

The “grain size” of a grain generally refers to the largest dimension of the grain. The largest dimension may be a diameter, a length, a width, or a height of a grain, depending on the geometry thereof. According to certain embodiments, the grains may be spherical, cubic, conical, cylindrical, needle-like, or any other suitable geometry.

According to certain embodiments, a relatively large percentage of the volume of the metal alloy is made up of small grains. For example, in some embodiments, at least 50%, at least 75%, at least 90%, at least 95%, at least 99%, or substantially all of the volume of the metal alloy is made up of grains having grain sizes of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm (and/or, in some embodiments, as small as 5 nm, as small as 2 nm, or as small as 1 nm). Other values are also possible.

According to certain embodiments, the metal alloy may have a relatively small average grain size. The “average grain size” of a material (e.g., a metal alloy) refers to the number average of the grain sizes of the grains in the material. According to certain embodiments, the metal alloy (e.g., a bulk and/or nanocrystalline metal alloy) has an average grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm. In certain embodiments, the metal alloy has an average grain size of as little as 25 nm, as little as 10 nm, at little as 5 nm, as little as 2 nm, as little as 1 nm, or smaller. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy has a small volume-average cross-sectional grain size. The “volume-average cross-sectional grain size” of a given cross-section of a metal alloy is determined by obtaining the cross-section of the object, tracing the perimeter of each grain in an image of the cross-section of the object (which may be a magnified image, such as an image obtained from a transmission electron microscope), and calculating the circular-equivalent diameter, D_(i), of each traced grain cross-section. The “circular-equivalent diameter” of a grain cross-section corresponds to the diameter of a circle having an area (A, as determined by A=πr²) equal to the cross-sectional area of the grain in the cross-section of the object. The volume-average cross-sectional grain size (G_(CS,avg)) is calculated as:

$G_{{CS},{avg}} = \left( \frac{\Sigma_{i = 1}^{i = n}D_{i}^{3}}{n} \right)^{1/3}$

where n is the number of grains in the cross-section and D_(i) is the circular-equivalent diameter of grain i.

According to certain embodiments, at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm. In certain embodiments, at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy has a volume-average cross-sectional grain size of as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy (that, optionally, intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, smaller than or equal to 10 nm (and/or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller); and at least a second cross-section of the metal alloy that is orthogonal to the first cross section (that, optionally, intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, smaller than or equal to 10 nm (and/or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller). Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy (that, optionally, intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm (and/or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller); at least a second cross-section of the metal alloy that is orthogonal to the first cross section (that, optionally, also intersects the geometric center of the metal alloy, or otherwise) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm (and/or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller); and at least a third cross-section of the metal alloy that is orthogonal to the first cross-section and that is orthogonal to the second cross-section (that, optionally, also intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 20 nm, or smaller than or equal to 10 nm (and/or as small as 25 nm, as small as 10 nm, as small as 5 nm, as small as 2 nm, as small as 1 nm, or smaller).

In some embodiments, the metal alloy comprises grains that are relatively equiaxed. In certain embodiments, at least a portion of the grains within the metal alloy have aspect ratios of less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and, in some embodiments, down to 1). The aspect ratio of a grain is calculated as the maximum cross-sectional dimension of the grain which intersects the geometric center of the grain, divided by the largest dimension of the grain that is orthogonal to the maximum cross-sectional dimension of the grain. The aspect ratio of a grain is expressed as a single number, with 1 corresponding to an equiaxed grain. In some embodiments, the number average of the aspect ratios of the grains in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and, in some embodiments, down to 1).

Without wishing to be bound by any particular theory, it is believed that relatively equiaxed grains may be present when the metal alloy is produced in the absence (or substantial absence) of applied pressure (e.g., via a pressureless or substantially pressureless sintering process).

In certain embodiments, the metal alloy comprises a relatively low cross-sectional average grain aspect ratio. In some embodiments, the cross-sectional average grain aspect ratio in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and, in some embodiments, down to 1). The “cross-sectional average grain aspect ratio” of a metal alloy is said to fall within a particular range if at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy is made up of grain cross-sections with an average aspect ratio falling within that range. For example, the cross-sectional average grain aspect ratio of a metal alloy would be less than 2 if the metal alloy includes at least one cross-section that intersects the geometric center of the metal alloy and in which the cross-section is made up of grain cross-sections with an average aspect ratio of less than 2. To determine the average aspect ratio of the grain cross-sections from which the cross-section of the metal alloy is made up (also referred to herein as the “average aspect ratio of grain cross-sections”), one obtains the cross-section of the metal alloy, traces the perimeter of each grain in an image of the cross-section of the metal alloy (which may be a magnified image, such as an image obtained from a transmission electron microscope), and calculates the aspect ratio of each traced grain cross-section. The aspect ratio of a grain cross-section is calculated as the maximum cross-sectional dimension of the grain cross-section (which intersects the geometric center of the grain cross-section), divided by the largest dimension of the grain cross-section that is orthogonal to the maximum cross-sectional dimension of the grain cross-section. The aspect ratio of a grain cross-section is expressed as a single number, with 1 corresponding to an equiaxed grain cross-section. The average aspect ratio of the grain cross-sections from which the cross-section of the metal alloy is made up (AR_(avg)) is calculated as a number average:

${AR_{avg}} = \frac{\Sigma_{i = 1}^{i = n}AR_{i}}{n}$

where n is the number of grains in the cross-section and AR_(i) is the aspect ratio of the cross-section of grain i.

According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio falling within a particular range (e.g., any of the ranges described elsewhere herein) has a first cross-section intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range, and at least a second cross-section—orthogonal to the first cross-section—intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range. For example, according to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a cross-section that intersects the geometric center of the metal alloy having an average aspect ratio of grain cross-sections of less than 2 and at least a second cross-section—orthogonal to the first cross-section—intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections of less than 2.

According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio falling within a particular range (e.g., any of the ranges described elsewhere herein) has a first cross-section intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range; a second cross-section—orthogonal to the first cross-section—intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range; and at least a third cross-section—orthogonal to the first cross-section and the second cross-section—intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range. For example, according to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a first cross-section that intersects the geometric center of the metal alloy having an average aspect ratio of grain cross-sections of less than 2, a second cross-section—orthogonal to the first cross-section—intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections of less than 2, and at least a third cross-section—orthogonal to the first cross-section and the second cross-section—intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections of less than 2.

According to certain embodiments, the grains within the metal alloy can be both relatively small and relatively equiaxed. For example, according to certain embodiments, at least one cross-section (and, in some embodiments, at least a second cross-section that is orthogonal to the first cross-section and/or at least a third cross-section that is orthogonal to the first and second cross-sections) can have a volume-average cross-sectional grain size and an average aspect ratio of grain cross-sections falling within any of the ranges outlined above or elsewhere herein.

Certain of the metal alloys described herein can have high relative densities. In some such embodiments, the metal alloys have high relative densities while maintaining their nanocrystalline character.

The term “relative density” refers to the ratio of the experimentally measured density of the metal alloy and the maximum theoretical density of the metal alloy. The “relative density” (ρ_(rel)) is expressed as a percentage, and is calculated as:

$\rho_{rel} = {\frac{\rho_{measured}}{\rho_{maximum}} \times 100\%}$

wherein ρ_(measured) is the experimentally measured density of the metal alloy and ρ_(maximum) is the maximum theoretical density of an alloy having the same composition as the metal alloy.

In some embodiments, the metal alloy (e.g., a sintered metal alloy, a nanocrystalline metal alloy, and/or a bulk metal alloy) has a relative density of at least 80%, at least 85%, at least 90%, at least 92%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% (and/or, in certain embodiments, up to 99.8%, up to 99.9%, or more). In some embodiments, the nanocrystalline alloy has a relative density of 100%. Other values are also possible.

According to certain embodiments, the metal alloy is fully dense. As utilized herein, the term “fully dense” (or “full density”) refers to a material with a relative density of at least 98%. According to certain embodiments, the relative density of the metal alloy may impact other material properties of the metal alloy. Thus, by controlling the relative density of the metal alloy, other material properties of the metal alloy may be controlled.

According to certain embodiments, metal alloys described herein can be substantially stable at relatively high temperatures. A metal alloy is said to be “substantially stable” at a particular temperature when the metal alloy includes at least one cross-section intersecting the geometric center of the alloy in which the volume-average cross-sectional grain size (described above) of the cross-section does not increase by more than 20% (relative to the original volume-average cross-sectional grain size) when the metal alloy is heated to that temperature for 24 hours in an argon atmosphere. One of ordinary skill in the art would be capable of determining whether a metal alloy is substantially stable at a particular temperature by taking a cross-section of the article, determining the volume-average cross-sectional grain size of the cross-section at 25° C., heating the cross-section to the particular temperature for 24 hours in an argon atmosphere, allowing the cross-section to cool back to 25° C., and determining—post-heating—the volume-average cross-sectional grain size of the cross-section. The metal alloy would be said to be substantially stable if the volume-average cross-sectional grain size of the cross-section after the heating step is less than 120% of the volume-average cross-sectional grain size of the cross-section prior to the heating step. According to certain embodiments, a metal alloy that is substantially stable at a particular temperature includes at least one cross-section intersecting the geometric center of the metal alloy in which the volume-average cross-sectional grain size of the cross-section does not increase by more than 15%, more than 10%, more than 5%, or more than 2% (relative to the original volume-average grain size) when the object is heated to that temperature for 24 hours in an argon atmosphere.

In some embodiments, the metal alloy is substantially stable at at least one temperature that is greater than or equal to 100 degrees Celsius (° C.). In certain embodiments, the metal alloy is substantially stable at at least one temperature that is greater than or equal to 700° C., greater than or equal to 800° C., greater than or equal to 900° C., greater than or equal to 1000° C., greater than or equal to 1100° C., greater than or equal to 1200° C., greater than or equal to 1300° C., greater than or equal to 1400° C., greater than or equal to 1500° C., greater than or equal to 1600° C., greater than or equal to 1700° C., greater than or equal to 1800° C., greater than or equal to 1900° C., greater than or equal to 2000° C., greater than or equal to 2100° C., greater than or equal to 2200° C., greater than or equal to 2300° C., greater than or equal to 2400° C., or greater than or equal to 2500° C. Other ranges are also possible.

Certain metal alloys described herein are sintered metal alloys. Exemplary sintering methods that may be used to produce metal alloys according to the present disclosure are described above.

Certain of the metal alloys described herein are stable against grain growth.

The metal alloy can, according to certain embodiments, be a bulk metal alloy (e.g., a bulk nanocrystalline metal alloy). A “bulk metal alloy” is a metal alloy that is not in the form of a thin film. In certain embodiments, the bulk metal alloy has a smallest dimension of at least 1 micron. In some embodiments, the bulk metal alloy has a smallest dimension of at least 5 microns, at least 10 microns, at least 25 microns, at least 50 microns, at least 100 microns, at least 500 microns, at least 1 millimeter, at least 1 centimeter, at least 10 centimeters, at least 100 centimeters, or at least 1 meter. Other values are also possible. According to certain embodiments, the metal alloy is not in the form of a coating.

In certain embodiments, the metal alloy occupies a volume of at least 0.01 mm³, at least 0.1 mm³, at least 1 mm³, at least 5 mm³, at least 10 mm³, at least 0.1 cm³, at least 0.5 cm³, at least 1 cm³, at least 10 cm³, at least 100 cm³, or at least 1 m³. Other values are also possible.

According to certain embodiments, the metal alloy comprises multiple phases. For example, in some embodiments, the metal alloy is a dual-phase metal alloy. In some cases, the metal alloy comprises a first solid phase rich in Mo and a second solid phase rich in a second metal. In other embodiments, the metal alloy is a single-phase metal alloy.

Certain embodiments are related to a metallic alloy based on molybdenum with a nanocrystalline microstructure, which is thermally stable. This alloy can be prepared from metallic powders by mechanical alloying, and then consolidated at high temperatures into a fully dense material, while retaining its nano-scale grain size. In accordance with certain embodiments, the dense nanocrystalline alloy is significantly stronger than a similar alloy which is not nanocrystalline.

According to certain embodiments, the alloys are based on molybdenum (Mo) and typically contain chromium (Cr) and/or tungsten (W) of varying compositions. They are prepared, according to some embodiments, by high-energy ball milling of elemental powders, which results in mechanical alloying (creating the alloy) and grain refinement (forming a nanocrystalline structure). In some embodiments, the alloy powders are then cold-compressed, and annealed in an inert atmosphere without any applied pressure. It is believed that, in accordance with certain embodiments, the addition of Cr stabilizes the grain boundaries so that the nanocrystalline structure is maintained during the annealing process. It is also believed that, in accordance with some embodiments, the addition of Cr helps accelerate the sintering (densification) process by forming a second phase during annealing. In some embodiments, alloys containing Mo and Cr may achieve full densification at temperature near 1450° C. This is lower than most conventional sintering methods for the production of molybdenum-based alloys. This may also provide the advantage of allowing the use of conventional equipment in producing alloys according to methods described here and may also reduce the energy required to produce parts.

Certain, although not necessarily all, of the embodiments described herein may have one or more advantages and/or improvements over existing methods, devices, and/or materials. According to some embodiments, methods described herein allow for creating fully dense bulk nanocrystalline parts with potentially complex shapes, in a scalable way. Alternative methods, such as severe plastic deformation methods (SPD) of dense, coarse-grained material, are believed to be generally not as scalable and are believed to be generally limited to simple part shapes. Additionally, certain of the methods described herein allow for sintering the powder without applied pressure during heating which greatly simplifies the processing route.

Certain articles, systems, and/or methods described herein can have any of a variety of commercial applications and/or may be particularly economically attractive. For example, certain of the alloys described herein can be made using much smaller amounts of energy (due to the low temperature and low pressure processing) than would be required for other types of molybdenum-containing alloys. Also, in accordance with certain embodiments, bulk metal parts (e.g., nanocrystalline metal parts) can substitute any structural metallic parts in commercial applications, as they may provide significantly improved mechanical properties. The molybdenum alloys described herein, in accordance with some embodiments, can replace conventional molybdenum alloy parts in construction, the auto, aerospace, and nuclear industries, and the like. In some embodiments, if their increased strength is not required, they can be used to reduce weight. For example, in accordance with certain embodiments, a thinner panel may provide the same engineering properties as a thicker one made of a conventional alloy. In some embodiments, alloys described herein can be used to provide both increased strength and weight reduction.

Certain of the alloys described herein may also be advantageous in high-temperature structural materials, such as in nuclear thermal propulsion. In some embodiments, alloys may possess a high enough melting temperature so they may operate at high temperatures (e.g., temperatures of at least about 2500° C.) for at least short periods of time (e.g., at least 1 minute, at least 10 minutes, or more). Additionally, certain embodiments of the alloys described herein may have a low neutron absorption cross-section, making them particularly suitable for use in a nuclear reactor.

The following example is intended to illustrate certain embodiments of the present invention but does not exemplify the full scope of the invention.

EXAMPLE

This example describes the enhanced sintering of Mo-based alloys. In certain embodiments, this set of alloys can work as structural material in nuclear thermal propulsion. Certain of the alloys described herein can be sintered at low temperatures, rapidly, and/or without the need for an applied pressure during the sintering process. In some such embodiments, the alloys can also have a high enough melting temperature such that it can operate at temperatures of up to 2500° C. (e.g., for at least short periods of time) and/or have an acceptable neutron absorption cross-section for use in a nuclear reactor.

Molybdenum is a viable candidate for a structural material in various nuclear reactor applications. Molybdenum has a relatively low neutron absorption cross-section which means that the neutrons released during the nuclear reaction stay contained in the reactor. These neutrons then react with the nuclear fuel, sustaining the nuclear reactions that allow the reactor to function. Furthermore, molybdenum has a high melting temperature allowing it to remain structurally stable in the high temperature environment of a nuclear reactor. Finally, it has high thermal conductivity which is useful for transferring heat to a working fluid. These factors make molybdenum alloys promising candidates for use in nuclear reactor designs including nuclear thermal propulsion systems.

Pure molybdenum, however, is generally not a suitable material for producing the complex components needed for advanced nuclear technologies such as nuclear thermal propulsion. It is generally difficult to produce parts from molybdenum due to its high melting temperature. Sintering pure molybdenum also generally requires a high applied pressure to achieve full density which limits the complexity of components that can be produced from it. Therefore, it would be useful to design a molybdenum alloy to facilitate pressureless sintering at lower temperatures. Furthermore, while molybdenum does have a high melting temperature, nuclear thermal propulsion requires operation at temperatures just above the practical operating range of pure molybdenum. Therefore, alloying molybdenum to increase its melting temperature would further increase its utility.

In certain embodiments, molybdenum alloys can be designed such that rapid sintering (which can involve nanophase separation sintering) is achieved. This example discloses certain molybdenum alloys that experience rapid, low temperature, pressureless sintering, via nanophase separation sintering. In this example, the alloying element chosen to promote nanophase separation sintering in molybdenum is chromium. Chromium has been observed to segregate into a secondary phase upon heating. In addition, chromium has a lower surface energy than molybdenum and therefore preferentially segregates to the surface of the powder particles to form necks between them. In addition, chromium allows for rapid diffusion of molybdenum through the chromium necks to promote rapid densification. These steps in the sintering process can be seen in FIG. 4.

To make the alloy, pure molybdenum and chromium powder were combined using mechanical alloying. Most testing employed a ratio of ˜15 at % chromium and the rest molybdenum (or Mo15Cr). This produced a metal powder (on the order of 1 micron in diameter) shown in FIG. 5 that was supersaturated (the molybdenum and chromium were evenly dispersed) and nanocrystalline with a grain size on the order of 10 nm. This powder was then pressed and formed into the shape of a part (or a green body) to ensure that the powder particles are in contact with each other. When the green body was raised to a higher temperature, the chromium atoms diffused to the surface of the powder particles. These atoms segregated out of the powder particle and formed a solid chromium phase on the surface of the powder particles. This separated phase formed between particles created necks between them as seen in FIG. 7. These necks acted as rapid diffusion pathways between particles, allowing material to flow through the system and further fill in the gaps between particles. This allowed the green body to sinter and reach >98% relative density. The resulting alloy was one that experienced the onset of sintering at temperatures as low as 750° C. and reached final density at temperatures as low as ˜1450° C. as seen in the microstructure in FIG. 6 and in the densification curve in FIG. 9.

In order to create a molybdenum-based alloy with a higher melting temperature, tungsten was included in the alloy. Tungsten has a high melting temperature and forms a solid solution with molybdenum. It is believed that, because of this, the melting temperature of the alloy can be tuned by increasing the quantity of tungsten the material is alloyed with. For example, an alloy that is 60 at % molybdenum, 25at % tungsten, and 15at % chromium (Mo25W15Cr) should exhibit a melting temperature 100 degrees higher than pure molybdenum. Similar to the Mo15Cr alloys tested, greater than 98% relative density was achieved in samples heated to only 1450° C. with no applied pressure as seen in the microstructure in FIG. 8 and the densification curve in FIG. 10.

The final melting temperature of the material remained high because the chromium phase that formed to accelerate sintering re-dissolved into the bulk molybdenum-based or molybdenum-tungsten-based material instead of melting at a lower temperature. While the addition of chromium did impact the melting temperature, the addition of more tungsten can be used to compensate for this.

The following processing steps were used to make the alloys:

-   -   1. The elemental molybdenum, chromium, and tungsten powders were         mechanically alloyed via ball-milling     -   2. The powder was formed into the desired shape (in the lab, the         powders were pressed into a pellet, but actual final parts may         have other shapes and sizes)     -   3. The pressed powder was placed into a furnace with a         controlled atmosphere. In this set of examples, Argon was used,         but other gases may work.     -   4. The pressed powder was heated to the desired temperature         range. The initial stages of sintering were observed near         750-800° C., and final density was achieved at temperatures near         1450° C.

The alloys described in this example can provide one or more of the following advantages. The alloys described in this example achieved full densification at temperatures near 1450° C. This is lower than most traditional sintering methods for the production of molybdenum-based alloys. This is beneficial because it allows more traditional equipment to be used in producing these alloys. Also, this reduces the energy required to produce parts.

The alloys described in this example were not held at their sintering temperatures for very long. This is beneficial for reducing the amount of energy required to produce parts. Also, faster part sintering is beneficial for having a higher throughput of parts that can be used or sold by the entity making them.

The alloys described in this example could be sintered without the application of external pressure. This means that items can be made from this alloy using simpler tools than similarly capable alloys. Furthermore, more complex geometries can be made from these alloys, making them useful for creating new objects with very specific functions.

The alloys described in this example can withstand high temperatures while exhibiting rapid sintering. Some methods that promote densification in materials result in the production of a secondary, low melting temperature phase that prevents the resulting material from being used at higher temperatures. In contrast, in the alloys described in this example, the secondary phase that forms re-dissolved back into the base alloy, and the melting temperature remained high.

For the alloys described in this example, the material stayed solid during the entire sintering process, so the shape change during the process was very limited. This is important when making parts with specific tolerances for final part geometries. Some accelerated sintering techniques result in parts deforming during the sintering process before reaching full density (specifically those where a liquid forms).

For the alloys described in this example, the melting temperature and nuclear absorption properties are tunable for the desired operating conditions of the final product. Tungsten alone does not have the neutron absorption properties necessary to be used a structural material in a nuclear reactor (specifically one meant to contain the reaction). Molybdenum alone cannot withstand the high temperatures of some advanced nuclear reactor designs. Using both of these elements to create an enhanced combination of high melting temperature and acceptable neutron absorption capabilities will facilitate the production of novel nuclear systems.

For the alloys described in this example, the production of the powders is industrially scalable. Mechanical alloying through ball milling is a common industrial method that is easy to scale from laboratory quantities (a few grams) to industrially relevant quantities (many kilograms). Other methods of accelerating sintering such as producing nano-sized powders are generally difficult to scale to commercially viable levels.

It is also believed that these are the only molybdenum-based alloys designed to exhibit nanophase separation sintering. The molybdenum-based alloys described herein have a variety of potential commercial applications. For example, in certain cases, the molybdenum-based alloys (e.g., also with chromium and/or tungsten) can be used in nuclear thermal propulsion. A nuclear thermal propulsion engine generally requires structures that exhibit a sufficiently low neutron absorption cross-section while being able to operate at temperatures of around 2500° C. Furthermore, the molybdenum-based alloys can be advantageous in producing complicated geometries needed to maximize surface area and create through channels for more efficient heat transfer to the propellant. Certain of the molybdenum-based alloys described herein co-optimize these properties in ways that no other alloys are believed to be capable of.

Certain of the molybdenum-based alloys described herein can be incorporated into nuclear thermal propulsion systems (e.g., for deep space missions and a possible manned missions to Mars). Certain of the molybdenum-based alloys described herein facilitate the extrusion of components through traditional manufacturing techniques. The molybdenum-based alloys described herein may also provide a pathway to 3D print novel components with complex geometries (e.g., for more specialized space craft). Certain of the molybdenum-based alloys described herein could also be utilized for high temperature application in novel nuclear reactors (e.g., fission and/or fusion nuclear reactors).

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and/or methods, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03. 

1. A method of forming a metal alloy, comprising: sintering particles comprising molybdenum (Mo) and a second element to produce the metal alloy, wherein: Mo is the most abundant element by atomic percentage in the metal alloy, and the metal alloy has a relative density of at least 80%.
 2. A method of forming a metal alloy, comprising: sintering particles comprising molybdenum (Mo) and chromium (Cr) to produce the metal alloy.
 3. The method of claim 2, wherein Mo is the most abundant element by atomic percentage in the metal alloy.
 4. The method of claim 3, wherein the metal alloy has a relative density of at least 80%.
 5. The method of claim 1, wherein the second element is chromium.
 6. The method of claim 1, wherein the second element is palladium (Pd).
 7. The method of claim 1, further comprising a third element.
 8. The method of claim 7, wherein the third element is present in the metal alloy in an amount of from 0.5 at % to 40 at % of the metal alloy.
 9. The method of claim 7, wherein the second element and the third element exhibit a miscibility gap.
 10. The method of claim 7, wherein the third element is tungsten (W).
 11. The method of claim 7, wherein the third element is tantalum (Ta).
 12. The method of claim 1, wherein the melting point of the metal alloy is at least 2,500° C.
 13. The method of claim 1, wherein the metal alloy has a neutron absorption cross-section of no greater than 18 barn.
 14. The method of claim 1, wherein Mo is present in the metal alloy in an amount of at least 50 at %.
 15. The method of claim 1, wherein the Mo and the second element exhibit a miscibility gap.
 16. The method of claim 1, wherein the metal alloy is nanocrystalline.
 17. (canceled)
 18. The method of claim 1, wherein the metal alloy is a bulk metal alloy.
 19. The method of claim 1, wherein the metal alloy is substantially stable at a temperature of at least 2500° C.
 20. (canceled)
 21. The method of claim 1, wherein the metal alloy is rich in the second element at grain boundaries of the metal alloy.
 22. A metal alloy, comprising: molybdenum (Mo); and a second element; wherein: Mo is the most abundant element by atomic percentage in the metal alloy, and the metal alloy has a relative density of at least 80%. 23-43. (canceled) 